SQL Server, ServicerBroker for asynchronized execution of stored procedures - sql

Question

I followed this tutorial:
https://gallery.technet.microsoft.com/scriptcenter/Using-Service-Broker-for-360c961a
and it is working for me,
However,
I don't understand some thing:
At PROCEDURE proc_BrokerTargetActivProc we have infinite loop: WHILE (1=1). Why ? After all, during creating queue we bind messages with this procedure:PROCEDURE_NAME = proc_BrokerTargetActivProc.
In addition, I am not sure If I correctly understand way of working it:
ExecuteProcedureAsync push to queue message with name of procedure to execute.
What now ? How does it work that BrokerTargetActivProc will be called with exactly one message ?
What about parameter MAX_QUEUE_READERS = 5 ?
Thank in advance,
Regards

Answer 1

You have three questions here.
Q. "Why do we have an infinite loop in the activation procedure?"
A. The idea here is that there is some non-zero cost for starting a procedure. If you have a bunch of messages on the queue, having your already executing procedure handle them is cheaper than executing the procedure for each message.
Q. How will the activation procedure be called with only one message?
A. That is an implementation detail of the way that BrokerTargetActivProc is written. Specifically, the RECEIVE TOP(1) statement. In my environment, I receive multiple messages off of the queue at once (e.g. RECEIVE TOP(1000)). That choice (and the implications of that choice) is up to you.
Q. What about parameter MAX_QUEUE_READERS = 5?
A. In order to fully appreciate this, a reading of this article is useful. It outlines when activation occurs on a service broker queue. Having MAX_QUEUE_READERS be greater than one says that you're allowing the server to have more than one process getting messages at a time. This would be useful in the case where you have a bunch of messages come in in a short period of time and you want to increase throughput by having multiple executions of your activation procedure active at once to run through those messages.
Follow-up questions from the comments:
Q: Who is calling the BrokerTargetActivProc procedure?
A: The procedure is called when activation is deemed to be necessary (see the article linked to above). As far as the executing context, you set that when you set the procedure as the activation procedure for the queue. For instance, if I wanted it to execute as foo_user, I'd do:
alter queue [TASK_QUEUE] with activation (
procedure_name = `BrokerTargetActivProc`,
execute as 'foo_user'
);
Q: How do you pass parameters to the activation procedure?
A: You don't. The point of the activation procedure is to de-queue messages and process them. So, all of the information should be in the message (which may drive queries etc).
Q: What about error handling?
A: You have to be careful here. Anything that causes a receive statement to rollback can trigger what is called poison message handling. That said, what I do is I wrap the receive and subsequent processing of a message in a try/catch block in the activation stored procedure and in the catch, I put the message into a table for later investigation. But how you handle errors and what you do with the messages that caused them is up to you!